1. Field of the Invention
The present invention relates to a method and apparatus for detecting and adjusting the position of a substrate on a robot blade.
2. Background of the Related Art
A common configuration for processing equipment utilizes a number of different processing chambers accessible from a central chamber, known as a transfer chamber. Typically, transfer of a substrate between the various processing chambers is performed by a robot disposed in the transfer chamber. To accommodate the high throughput requirements of semiconductor processing, the robots are adapted for accurate, high-speed movement. The robot includes a substrate seating surface for supporting a substrate thereon and is capable of rotation and extension. Clamping mechanisms are typically used to secure the substrate to the substrate seating surface and prevent slippage which can result in damage.
An exemplary frog-leg type robot 10 is shown in FIG. 1. The robot 10 comprises a four-bar linkage 12 mounted to a pair of central hubs 14 (only one shown) which may be actuated by stepper motors (not shown). A robot blade 16 connected to the linkage 12 is adapted to support a substrate 18 thereon. Clamp fingers 20 are provided to secure the substrate 18 during movement of the blade 16. In operation, the hubs 14 are rotated by the stepper motors to cause linear and rotational actuation of the robot blade 16. Rotation of the hubs 14 in the same direction causes rotation of the blade 16 while rotation of the hubs 14 in opposite directions causes extension and retraction of the blade 16. When a substrate 18 is disposed on the blade 16, the clamp fingers 20 are actuated toward the edge of the substrate 18 to urge the substrate 18 against a shoulder 22, or "shoe." Thus, the shoulder 22 and the clamp fingers 20 cooperate to hold the substrate 18 during movement of the robot 10. FIG. 2 shows a substrate 18 properly positioned between the shoulder 22 and the clamp fingers 20.
Normally, stepper motor driven robots under computer control, such as the one shown in FIG. 1, are capable of repeatedly transporting substrates through a processing system with great speed and precision. However, the effectiveness of such substrate handling techniques can be greatly diminished if the initial position of the substrate is not known. For example, FIG. 3 shows a substrate 18 improperly positioned on the blade 16, wherein a portion of the substrate 18 is disposed on the shoulder 22. Such positioning of the substrate may occur during operation for various reasons. For example, the lift mechanism (lift pins) which deposits the substrate onto the blade may be improperly adjusted and vibrate, thereby causing the substrate to "walk" on the lift mechanism. Other causes include the effects of processing on the substrate due to gases delivered to the backside of the substrate and the plasma used during deposition of a material onto the substrate. Regardless of the cause for improper substrate positioning, upon actuation of the blade 16, an improperly positioned substrate 18 will likely slip from the blade 16 and be damaged. The likelihood of slippage is particularly great during rotation of the blade 16. Because current technology does not provide an accurate method of determining whether a substrate is securely clamped, substrates fall from the blade causing damage to the substrate, thereby requiring the system to be halted for operator intervention. The problems associated with unclamped substrates are heightened by use of increasingly faster robots.
Another problem associated with substrate transfer robots is the potential for misalignment of a properly clamped substrate in a chamber. In semiconductor processing, it is desirable to know the exact location of a substrate relative to the robot blade so that the substrate can be precisely positioned at an optimum location at a final destination such as within a processing chamber. Knowledge of the substrate position allows repeatably positioning substrates in a chamber at substantially the same location, thereby maximizing the effectiveness of the processing onto the desired surface area of the substrate to be processed. Ideally, clamped substrates being transferred by the robot are situated at the substrate's nominal position within the pocket of the blade. In practice, however, substrates are not always disposed at or substantially near the nominal position causing the robot to deposit the substrate in the chamber at a position displaced from the intended destination. Therefore, current methods utilize centerfinding techniques to determine the centerpoint of each substrate and position the substrate accordingly, thereby ensuring that each substrate is positioned uniformly relative to the known centerpoints.
While methods for substrate centerfinding are known, current technology does not provide a method or apparatus for detecting the clamped or unclamped state of a substrate as well as allow for corrections in substrate positioning to ensure proper alignment in a process chamber. Further, known methods of centerfinding have several disadvantages resulting in reduced throughput and increased complexity and cost. For example, one known method comprises a bank of sensors and detectors disposed inside the vacuum environment of a processing system. A substrate is moved into the optical paths of the signals emitted by the sensors, thereby blocking the signals. Once the signals become blocked the output of the detectors switches states. The change in the output of the detectors is then used to calculate the center of the substrate. The requirement of multiple sensors is a disadvantage because of the cost and increased complexity of the system. Typically, such an arrangement is feasible only at one location in the processing system requiring substrates to be transported to the location of the bank of sensors each time centerfinding is to be performed, thereby limiting throughput. Further, by positioning the sensors inside the vacuum environment the sensors can outgas particles leading to contamination of the substrates. Thus, it would be preferable to perform the centerfinding on-the-fly, i.e., during the normal operating sequences of a robot in order to minimize the impact on throughput. It would also be preferable to limit the number of electronic sensing components and to position the components outside the vacuum environment of the processing chamber.
Other centerfinding techniques utilize a spindle type apparatus whereby the substrate is transferred to a spindle assembly and incrementally rotated to determine the centerpoint offset by geometric analysis. Such an arrangement is undesirable because the apparatus is separate and distinct from the processing system, thereby requiring additional steps and costs to the manufacturing process and inhibiting productivity.
Therefore, there is a need for an apparatus and method to determine the clamped or unclamped state of a substrate on a robot support member as well as allow for necessary corrections in the position of the substrate in a process chamber. Preferably the apparatus is positioned outside a vacuum environment of a processing chamber and is adapted to operate on-the-fly.